1. Field of the Invention
This invention relates to semiconductor processing equipment and, more particularly, to a downstream plasma reactor system employing an improved plasma tube sealing configuration, and to a method for configuring such a reactor system.
2. Description of the Related Art
Plasma processing is commonly used in semiconductor fabrication. One use for plasma processing is in the removal of layers formed on a substrate, typically by etching some or all of a particular layer. Plasma processing is often performed in single chamber reactor systems in which the plasma is generated exclusively in the chamber in which processing is carried out. Alternatively, downstream plasma reactor systems may be used that first convert gases into plasma in a plasma tube and then transport the plasma-generated reactive species downstream into the reaction chamber. These reactor systems can be used to avoid the radiation damage and resist hardening common in single chamber plasma reactor systems. And like single chamber plasma reactor systems, downstream plasma reactor systems can be used to create reactive species capable of etching layers of silicon dioxide, silicon nitride, aluminum, and various other materials commonly used in semiconductor fabrication.
A common use for downstream plasma reactor systems is in resist stripping, i.e., the removal of patterned photoresist after completion of an etch step. Resist stripping usually is carried out in an ashing process in which the resist is oxidized to a gaseous form and removed from the reaction chamber. Those downstream plasma reactor systems that are specifically configured for resist stripping are labeled downstream plasma strippers.
FIG. 1 presents a schematic view of an exemplary downstream plasma reactor system 100, the GaSonics L3500, which is commercially available from GaSonics International, San Jose, Calif. Downstream plasma reactor system 100 may be properly labeled a downstream plasma stripper because it is primarily configured to remove resist. Reactor system 100 includes a plasma tube 104. Plasma tube 104 is made up of an intake portion 106, a central portion 108, and a discharge portion 110. Gas source 102 is in gaseous communication with intake portion 106 of plasma tube 104. Plasma tube 104 is coupled to inlet conduit 112. Inlet conduit 112 is connected to reaction chamber 114. Plasma generating apparatus 111 is positioned adjacent to plasma tube central portion 108, and includes a power supply and a microwave generator. Outlet conduit 116 is connected to reaction chamber 114 and is in selective gaseous communication with vacuum pump 118.
During operation of downstream plasma reactor system 100, vacuum pump 118 may be used to evacuate gases from reaction chamber 114 and all conduits in gaseous communication with reaction chamber 114, including inlet conduit 112 and plasma tube 104. Gases may be introduced into plasma tube 104 from gas source 102 via intake portion 106. The desired amounts and proportions of gases supplied by gas source 102 may be regulated using one or more mass flow controllers. These gases are typically selected such that the reactive species generated upon plasma formation are appropriate for the particular process being performed. As the gases enter central portion 108, microwaves created by plasma generating apparatus 111 convert at least a portion of the entering gases into plasma. The plasma generated in central portion 108 subsequently passes into discharge portion 110. From discharge portion 110, the plasma is conveyed into inlet conduit 112. The plasma is transported through inlet conduit 112 into reaction chamber 114 to be used in processing.
FIG. 2 presents an expanded cross-sectional view of section A of reactor system 100. Section A includes parts of discharge portion 110 of plasma tube 104 and coupling portion 126 of inlet conduit 112. As shown in FIG. 2, discharge portion 110 may be subdivided into a first section 120, an expanded section 122, and a second section 124. Discharge opening 125 is defined at the end of second section 124. Scaling o-ring groove is defined within expanded section 122 and is configured to hold scaling o-ring 130. Sealing o-ring 130 is composed of an elastomeric material. Sealing o-ring 130 is configured to make a seal between plasma tube 104 and inlet conduit 112 sufficient to maintain the level of vacuum desired.
Sealing o-ring 130 should not only provide a good seal between plasma tube 104 and inlet conduit 112, but should be able to maintain such a seal over numerous operation cycles carried out over a sizable time period. To maintain a sufficient seal over repeated operation cycles, sealing o-ring 130 should possess ample resiliency. Good resiliency in sealing o-ring 130 is important because when reactor system 100 is under vacuum during an operation cycle, coupling portion 126 exerts substantial lateral force on the sealing o-ring. Then when the cycle is completed, the vacuum is released and the lateral force exerted by coupling section subsides. A sufficiently resilient sealing o-ring 130 is able to compress during operation of a cycle and then return to its original shape after the cycle is complete. Consequently, the quality of the seal may be maintained over numerous operation cycles.
In addition, it is desirable that the time between failures of sealing o-ring 130 be extended as long as is reasonably possible. Replacing sealing o-ring 130 requires the purchase of a new o-ring and necessitates the expenditure of limited employee time. Over time, the total value of the production lost during these replacement periods can become quite substantial.
Unfortunately, the operating conditions of reactor system 100 can greatly reduce the amount of time between failures of sealing o-ring 130. One explanation for this outcome is the presence of numerous reactive species in the gases exiting the plasma tube. Most of these reactive species will pass directly into the inlet conduit, but some end up in contact with sealing o-ring 130. While these plasma-generated reactive species do not substantially erode the fused quartz of which plasma tube 104 is constructed, other elements of the plasma system, such as sealing o-ring 130, are often constructed of materials more susceptible to such erosion. Furthermore, resist stripping often incorporates hydrogen- and oxygen-containing plasmas that have a particularly pronounced ability to degrade many commonly used sealing materials. As such, the chemical resistance of sealing o-ring 130 to plasma-generated radicals can greatly influence the average time between failure of such an o-ring.
In an attempt to resolve these problems, numerous chemically resistant elastomers have been used for sealing o-ring 130. One of these is Viton.RTM., a fluoroelastomer commercially available from DuPont Dow Elastomers, Wilmington, Del. Viton.RTM. has good resiliency, and is suitable for use in vacuum operations. But while Viton.RTM. and similar fluoroelastomers possess some chemical resistivity, they generally are still relatively susceptible to erosion by plasma-generated reactive species. Over time, the constant attack of these reactive species can break off portions of sealing o-ring 130. These portions may then be swept into the reaction chamber 114 where they can cause damage serious enough to prevent the formation of functioning integrated circuits. Eventually, plasma-generated reactive species can even erode enough of sealing o-ring 130 to cause its complete failure. When used as sealing o-ring 130 in reactor system 100, such o-rings often fail in less than three days--an undesirably short time period.
Increased success has been obtained using materials such as Kalrez.RTM. (a perfluoroelastomer commercially available from DuPont Dow elastomers) and Chemraz.RTM. (a perfluoroelastomer commercially available from Green, Tweed & Co, Kulpsville, Pa. Because of the enhanced chemical resistivity of these materials, a sealing o-ring 130 made of such perfluoroelastomers typically lasts longer than a sealing o-ring 130 made of a fluoroelastomer like Viton.RTM.. These materials are more expensive than Viton.RTM., however, and still often fail in only 7 days of operation.
One type of o-ring that has been able to increase the time before erosion-induced failure of sealing o-ring 130 is an o-ring encapsulated with Teflon.RTM. (a fluorocarbon polymer commercially available from E.I. du Pont de Nemours and Company). Teflon.RTM. encapsulated o-rings typically include a Teflon.RTM. jacket that surrounds an elastomer core. These o-rings are substantially more resistant to erosion by plasma-generated reactive species than the elastomeric materials mentioned above.
Unfortunately, Teflon.RTM.-encapsulated o-rings are not well suited for use as sealing o-ring 130. The Teflon.RTM. jacket of these o-rings makes these o-rings less resilient than elastomeric o-rings. The inflexibility of Teflon.RTM.-encapsulated o-rings compared to o-rings composed of elastomeric materials can increase the difficulty of coupling plasma tube 104 and inlet conduit 112. And because of the relative lack of resiliency in the Teflont.RTM. jacket, a Teflon.RTM.-encapsulated o-ring may not be able to fully return to its original shape after being compressed during an operation cycle. Over numerous compression and expansion cycles, a Teflon.RTM.-encapsulated sealing o-ring may become substantially deformed. The discrepancy between the sealing o-ring's original shape and its deformed shape can significantly reduce the sealing ability of the o-ring. Eventually, a Teflon.RTM.-encapsulated sealing o-ring 130 may become so deformed that it can no longer make the necessary seal. Even worse, the buildup of microstresses in the Teflon.RTM. jacket of the o-ring can cause the o-ring jacket to crack, potentially creating an immediate loss of vacuum.
Therefore, it would be desirable to develop a downstream plasma reactor system with an improved plasma tube sealing configuration. The improved design should significantly extend the mean time between failure of the seal between the plasma tube and an inlet conduit to a reaction chamber. This improved sealing configuration should be able to be incorporated without significantly increasing the install difficulty or reducing the seal quality.